# Dreyer Calculator

The Dreyer Calculator as Proposed
Visible are the means of setting the Air Density, Range Rate, Plot Range, and Wind along the line of fire. The range spotting correction is read off of scale H. Only the grayish piece N is peculiar to the gun system in use, containing a "Wind Curve" (label obscured in this image by compositing) at the top, which affects the impact of Air Density and Wind on the output and a "Time of Flight Curve" at the bottom which affects the Range Rate's contribution.[1]
Dreyer Calculator showing action[2]
Dreyer Calculator, in use[3]

The Dreyer Calculator was a British Fire Control Instrument that determined the aggregate effect on gun range attributable to a number of factors. It was invented by John Tuthill Dreyer, brother to Captain Frederic Charles Dreyer and worked in concert with his brother's fire control table.

## Genesis and Nature

The Dreyer calculator was a small calculating board that eventually enjoyed a place on the bulkhead of Royal Navy Transmitting Stations. It calculated several separate factors that would cause the clock range and gun range to differ and summed them together to produce an aggregate spotting correction for range.

It appeared in 1908, along with definite mention of its adoption and manufacture.[4]

## Inputs and Mechanical Design

A wooden case with a carry handle was envisioned, and the full size drawing seems to hint at dimensions of 33cm by 55cm with a depth of 65mm or so.

The calculator was customized to a given Gun System by the choice of which backplate (bearing the range scale) was fitted. Milled curves along these backplates expressed ballistic relationships taken from the Range Table, such as range to time-of-flight.

The inputs were:

• Plot Range (between 5,000 and 30,000 yards in the 1920 models)
• Range Rate (1908's drawing indicated in +/- 50 knots, annotated that this should move to the new yards per minute norm. The 1920 design seemed to vary between about +/- 15000 yards per minute).
• Air Density (difference from the standard atmosphere of the range table)
• Wind along the line of fire
• Temperature (apparently added after 1908)[5]

An additional input plate was available for use on the lower curve, but by 1920, no use of it had been devised, and it was blank and to be left zeroed.

By altering the plot range with a large knob, the backplate's "time-of-flight vs range curve" would interact with the range rate and a "wind vs range curve" would influence the degree to which air density, temperature and wind along affected the output. A pair of vertical strips would sum these influences to yield a correction to range within a range of +/- 3,000 yards.[6]

## Application and Use

Vexingly, after the Battle of Jutland, it was reported that the battlecruisers entirely neglected their calculators. The Battlecruiser Fleet Committee reported,[7]

Range Calculator. The range calculator was not used by any ship. This is doubtless due to opinions formed in peace practices at comparatively short ranges, and under easier conditions, when the corrects were consequently small. The committee consider that at the very much greater ranges and more difficult conditions that we now have to contend with, these corrections being greatly increased, must not be neglected if early hitting is to be established. This should be pointed out to the fleet, and the use of the range calculator insisted upon, all ships being supplied.

However, in incorporating other lessons of Jutland, it was also ordered that ships should all have wind vanes and anemometers to provide their calculator with wind-along data.[8]

In ships with Dreyer Fire Control Tables, this correction would be entered as a straddle correction[9] into the Spotting Corrector to be summed with clock range and accrued spotting corrections for range.

As the battle progressed, it was of course common for all the inputs (except air density) to change. It would be a fairly manual task for someone to keep the Dreyer calculator updated and to feed the changing result back to the Dreyer table's spotting corrector. I can imagine that ships lacking a tool similar to the spotting corrector would forego use of a Dreyer calculator and simply rely on spotting to account for these dynamics.

The Grand Fleet Dreyer Table Committee found that in practice the Fleet's Dreyer calculators' output was being handled in a variety of ways by the various personnel. Focusing on ships equipped with Mark IV tables and later:[10]

• nine ships had a worm shaft by which the calculator operator could shift the zero of the Spotting Corrector on the table, for follow-up by its operator
• five ships applied the correction only initially, relying on spotting corrections thereafter
• three ships applied the correction due to own ship's motion only, in deference to a lack of confidence in tracking enemy movement
• two ships did not use the calculator's indication at all

The calculator continued to have a role in British fire control alongside Mark III* and IV Dreyer tables into the 1930s, and probably into World War II.[11]

## Footnotes

1. (G. 4023/08) Fire Control, 1908, Enclosure XIV(b)
2. Manual of Gunnery Volume 2, 1920, Plate 21.
3. Manual of Gunnery Volume 2, 1920, Plate 22.
4. Fire Control, 1908, pp. 5-6, Enclosure XIV(b), (G. 4023/08)
5. Not depicted in the 1908 drawing, but clearly evident in Manual of Gunnery, Volume III, 1920.
6. The drawing from 1908 depicted +/- 2,000 yards, but is textually noted that 4000 would be the limit. However, the 1920 source indicates the 3000 yard figure as the final one chosen.
7. Grand Fleet Gunnery and Torpedo Orders. 167.
8. Grand Fleet Gunnery and Torpedo Orders. 167.
9. Handbook of Captain F. C. Dreyer's Fire Control Tables, 1918. p. 8.
10. Reports of the Grand Fleet Dreyer Table Committee, 1918-1919, p. 8.
11. Pamphlet on the Dreyer Tables Mark III*, 1930 (p.3 and Plate 1) and Pamphlet on the Dreyer Tables Mark 1V, 1930 (p.7 and Plate 2) in Guard Book for Pamphlets on Dreyer Tables, AL., citations thanks to John Brooks.

## Bibliography

• Admiralty, Gunnery Branch (1908). Fire Control, 1908. (G. 4023/08) C.B. 1126. At Admiralty Library, Portsmouth, United Kingdom.
• Admiralty, Gunnery Branch (1918). Handbook of Captain F. C. Dreyer's Fire Control Tables, 1918. C.B. 1456. Copy No. 10 at Admiralty Library, Portsmouth, United Kingdom.
• Admiralty, Gunnery Branch, (1919). Reports of the Grand Fleet Dreyer Table Committee, 1918-1919. Pub. No. C.B. 1533. The National Archives. ADM 186/241.